1. Technical Field
The present invention relates generally to integrated circuits, and more specifically to local interconnects and isolated contacts for interconnecting semiconductor devices.
2. Background Art
As semiconductor technology continues to evolve, a continuing trend is towards ultra large-scale integration with the fabrication of smaller and smaller integrated circuits with more and faster semiconductor devices.
Fabrication of an integrated circuit involves numerous processing steps. After doped regions have been deposited to form source/drain junctions within a semiconductor substrate and gates have been defined on the substrate, dielectric layers are deposited on the semiconductor devices and conductors are routed over the dielectric layers to connect to and fill openings formed through the dielectric layer to the source/drain junctions and gates. The entire process of routing and making connections is generally termed “metalization”. The term derives its origins from interconnect technology, where metals were the first conductors used, but encompasses both metals and conductive materials such as polysilicon. As the complexity of integrated circuit is increased, the complexity of metalization has also increased.
At the same time that the complexity of metallization has increased, multiple layers of interconnect structures have been have come into use as well as short distance interconnects at levels at or below the customary metallization layers. The latter are termed “local interconnects” and are a special form of interconnects for very short distances, such as between the gate and drain of an individual semiconductor device.
A commonly used technique for forming local interconnects is the damascene process. This process involves depositing a dielectric layer over the semiconductor device and then polishing the dielectric layer to make the layer planar. The layer is then patterned and etched to form openings down to the underlying gate or source/drain junctions. A conductor is then deposited in the openings and a chemical-mechanical polishing process (CMP) is used to damascene a conductor into dielectric layer to form the local interconnects and isolated contacts.
There are a number of problems with existing processes such as those caused by larger openings etching at a different rate than smaller openings. This means that the larger local interconnect contacts which have larger openings than the isolated contacts will be completed before the isolated contact openings are open to the source/drain junctions. This is especially true for very small isolated contact openings between very tightly spaced gates (especially at 0.18μ or lower).
Further, the process window for time for etching openings becomes very short when trying to create the different sized isolated contacts and local interconnects. This is especially true at 0.16μ or lower, where the local interconnects are three to four times larger than the isolated contacts. In these cases, the processes often result in gouging of the shallow trench isolations, which separate the semiconductor devices, and loss of isolating spacers.
Also, incomplete filling results in a void area, also known as a “keyhole,” that is formed within the metalization. This keyhole is detrimental because it can open up during further processing steps, where material which could corrode or corrupt the tungsten layer can make its way into the keyhole. Also, the void in the center of the conducting metalization layer in the contact causes an increase in contact resistance.
Solutions to problems of this sort have been long sought, but there has been no teaching or suggestion in the prior art how those having ordinary skill in the art could solve these problems.